Transmission elements, in particular coated optical fibers, used in telecommunication cables are typically protected, either individually or as a group, by buffering materials or elements.
For instance, one or more optical fiber, group, bundle or ribbon of optical fibers may be protected by a polymeric material in the form of a tube or of a flexible sheath. The optical fiber together with its protective element is generally referred to in the art as “optical unit”. An optical cable may contain a single optical unit or a plurality of optical units. Said single or plurality of optical units is generally referred to as the optical core of the cable. The optical core is in turn typically inserted into a protecting polymeric sheath.
U.S. Pat. No. 4,909,593 discloses an optical cable comprising several multiple-fiber optical units disposed within a tube made of a rigid, hardly shrinking plastic. Each of said unit consists of several optical fibers and of an envelope of soft plastic which can be easily removed with bare fingers. Suitable plastics are thermoplastic elastomers vulcanizable at room temperature, or soft thermoplastic elastomers such as polyesteramide copolymers, soft ethylene-propylene copolymers, or a styrene-butadiene rubber.
EP patent application no. 1 024 382 discloses a telecommunication cable comprising a flexible buffer tube made from a thermoplastic polyolefin elastomer having a modulus of elasticity below 500 Mpa at room temperature and a modulus of elasticity below 1500 Mpa at −40° C. Examples of suitable elastomers are ethylene-propylene copolymers, preferably with more than 10 percent of ethylene monomer, terpolymers containing propylene-ethylene, ultra low density polyethylene or ethylene-octene copolymers, preferably containing more than 10% by weight of octene monomer. The elastomer can also contain inorganic fillers for controlling physical parameters, such as mechanical properties and flame retardancy.
The article from G. Dallas et al., “Thermal and Mechanical Optimization of Easy-Access Flexible Buffering Materials”, 49th International Wire & Cable Symposium, 2000, pp. 357–361 discloses the use of mixtures of hard impact-resistant polypropylene and of a soft grade impact-resistant polypropylene, optionally in admixture with soft ethylvinylacetate copolymers, for making flexible optical buffer tubes. As mentioned in said article, improved access to the fiber bundle housed within the tube (as compared to a conventional PVC sheath) is obtained by an easier removal of the polypropylene sheath by tearing said sheath with a ripcord disposed within said sheath.
The Applicant has now found that by using a polymeric material comprising an heterophasic olefin copolymer having at least one amorphous phase and at least two different crystalline phase, it is possible to manufacture tubular elements for telecommunication cables which have improved tearableness.
Along the present description and claims, the term “tubular element” is intended to include within its meaning any element which has or can be disposed in a tubular form within the cable structure. Examples of such tubular elements are buffer tubes housing at least one transmission element or polymeric sheaths disposed to surround inner portions of a telecommunication cable, e.g. one or more buffer tubes. Said polymeric sheath is preferably in the form of a tube (e.g. extruded about said inner portion) or alternatively can be a tape disposed about said inner portion (thus taking a tubular form), either helically wrapped or preferably folded along its longitudinal direction about said inner portion.